1. Field of the invention
The present invention relates to improvement in a silicon nitride film deposition method, and more particularly to a method of depositing, by a photo-CVD (Chemical Vapor Deposition) technique, a silicon nitride expressed by Si.sub.3 N.sub.4-x (where 0.ltoreq..times.&lt;4) on a substrate.
2. Description of the prior Art
In semiconductor devices, a silicon nitride film deposited on a substrate is widely used as a dielectric thin film, insulating film or the like. In a semiconductor manufacturing process, a silicon nitride deposited on a substrate is widely employed as an impurity diffusion mask, etching mask or the like.
Conventionally, a plasma-CVD technique, a photo-CVD technique and so forth have been proposed for such silicon nitride film deposition on a substrate.
With the plasma-CVD technique, the silicon nitride film can be deposited on the substrate in a relatively low-temperature atmosphere without heating the substrate to high temperatures and consequently without inflicting thermal damages to the substrate. Therefore even if the substrate is a semiconductor substrate that has formed thereon a semiconductor element or interconnect structure, the semiconductor substrate and the interconnect structure are free from thermal damages. Accordingly, the plasma-CVD technique is suitable for use in semiconductor device fabrication including silicon nitride film deposition on a semiconductor substrate.
In the silicon nitride film deposition by the plasma-CVD technique, however, a plasma causes radiation induced damages to the deposited silicon nitride film or the underlying substrate due to the plasma bombardment thereof.
On the other hand, a photo-CVD technique also permits the silicon nitride film deposition on the substrate without the necessity of heating it to high temperatures, as is the case with the plasma-CVD technique. Therefore, this technique is also suitable for use in the semiconductor device fabrication including the silicon nitride film deposition on the substrate.
The photo-CVD technique utilizes light, and hence does not inflict the abovesaid damages to the deposited silicon nitride film or the underlying substrate.
With the conventional photo-CVD technique, a source gas is a gas mixture of monosilane (SiH.sub.4) and ammonia (NH.sub.3) gases and containing mercury vapor as a photo-sensitizer, and 254 nm ultraviolet light from a mercury lamp is used light irradiation.
Such a prior art photo-CVD method permits more effective optical excitation of the source gas for more effective chemical reaction thereof than in the case of the source gas containing no mercury vapor as a photo-sensitizer, ensuring the silicon nitride film deposition at a practically acceptable deposition rate.
However, the conventional photo-CVD technique presents a problem as its source gas contains the mercury vapor which pollutes the silicon nitride depositing device and its surroundings and damages workers' health.